Long-term immunity to antigens previously encountered through illness or vaccination is believed to reside in memory T and B lymphocytes that persist in a resting state over the lifetime of an individual. For CD4 T cells, which orchestrate adaptive immune responses to infectious agents, tumors, and self antigens, long-lived, resting memory T cells are believed to arise directly from a subset of short-lived, activated effector CD4 T cells that have reverted to the resting state. This linear progression of effector cells into resting memory T cells is favored, due to the phenotypic and functional similarities between effector and memory T cells. Recently, results from human and mouse systems has revealed the presence of effector-like T cells within the long-lived memory T cell pool, indicating heterogeneity of the memory T cell population. We have recently developed an activation profile assay that distinguishes between effector and memory CD4 T cells and also distinguishes subsets of long-lived mouse memory CD4 T cells that differ in expression of the CD62L homing receptor. Our findings suggest that the mouse memory CD4 T cell pool consists of effector-like memory cells and classical resting memory T cells. The long-term goal of the proposed research is to determine what roles these heterogeneous memory T cell subsets play in anamnestic immune responses. We hypothesize that these functionally distinct memory subsets determine the overall nature of the recall response and are differentially generated and maintained in vivo. To address this hypothesis and work toward achieving our long-term goal, our specific aims are to characterize these heterogeneous memory CD4 T cell subsets on the cellular and molecular level, and determine mechanisms for their generation and maintenance in vivo using mouse adoptive transfer models. The proposed research endeavors to dissect the complex nature of the long-lived T cells that mediate memory immune responses to different types of antigens.